There's no FDA rules about what "low-carb" means.  So legally, anyone
can claim anything is low-carb.

It *sometimes* makes a difference where carbs come from.  Most low-carb
authors agree you can discount fiber.  So if your total carb count for
something is 10 per serving, but 6 of that is from fiber, you can
"count" it as 4.

The "net carb" business loudly announced on packages of "low-carb"
foods not only subtracts fiber, but sugar alcohols.  That is a more
controversial stance, in that some people react to some sugar alcohols,
so it's hard to say how "low-carb" the addition of sugar alcohols is.

All starchy foods are high-carb.  Like... they're developing a
lower-carb potato - it's still *way* high carb.  Most of your root
veggies are very starchy.  Also all grain products, whether whole
grains, or breads or pasta or such.  The only "low-carb" of such things
are made mostly from fiber - bran, or fiber-based products like some
crispbreads or low-carb tortillas.

All whole meats are low-carb.  Prepared meats like lunch meats and
sausage and such might be low-carb or not, depending on what fillers
are added.

Generally, the highest-fat dairy products are the lowest-carb.  So...
heavy cream is low-carb and skim milk is higher carb.

Legumes are generally a tad high to fit on most low-carb diets, except
for black soybeans.  People on relatively higher low-carb diets can
probably fit more in.

Nuts vary, some being lower carb than others.

Most of your non-starchy veggies are relatively low-carb.  All your
alliums and carrots are exceptions, they tend to be higher carb than
the other non-starchy vegetables.

Melons and berries are the lowest carb fruits, but they're higher than
most of your non-starchy veggies.

Obviously, all your forms of sugar are out, natural or not.  Most of
your artifical sweetners are "bulked up" with carbs, nearly always in
those little packets, and even moreso in the stuff that is measured
like sugar.  Products made with them are often lower-carb because the
manufacturers don't have to add the bulking agents - DaVinci sugarless
syrups, for example, are virtually carb-free.

In general, a diet containing less than 100 grams of carb is considered
low-carb.  So technically, a meal with 25-30 grams of carbs would be OK
for *some* folks.

But most people on low-carb diets are eating at a much lower level than
100 grams/day.  So that same amount of carb that would be OK for a
minority of folks would be the whole daily budget of carbs for others.

I eat at around 40-50 grams/day... mostly because I do eat small
servings of the lower-carb fruits daily with cottage cheese, yogurt,
ricotta or cream cheese.  If I were not eating fruit, I'd be at 30-40
grams/day.

There's nothing tricky about it, but it depends on what you mean by "a
recipe."  If you're talking about something like a one-dish casserole
meal, it can afford to have more carbs per serving than a side dish
that will probably accompany other dishes that may also have carbs.
Also, what's low-carb enough for one person's eating plan may not be
acceptable to another's.  It's just not possible to put a number on it
like that.

However, 30gm/day is acceptable for most low-carb plans except for
Atkins Induction, so if your recipe comes in at under 10gm per meal
per person, I'd say it's safe to call it low-carb.  But the best thing
to do is to simply state carbs/serving on your recipe, and let people
decide for themselves.

Yes, a carb is a carb.  However, some people try to get their carbs
from sources which cause them to be absorbed into the blood stream
more slowly than others.  The long-term effect on ketosis and weight
loss should be the same, but the theory is that the slower absorption
helps to keep blood sugar from spiking in people who have problems
with that.

If it does, I'm sure it's wrong.  You're not talking averages when you
talk carbs, anyway; you're talking maximums.